It is known from DE 3 409 299 to control the speed of a three-phase motor by pulse width modulating the mains voltage via transistor driving stages which are controlled by a control circuit which is galvanically separated from the driving stages. The driving stages are adapted for reversing the phase to change the direction of rotation.
"Thyristor Control of A. C. Motors", JMD Murphy, Pergamon Press, 1973, chapter 9.3, pages 154-158, shows an example of a voltage controlled speed control of a three-phase induction motor. The control is carried out by feedback of the actual speed which is compared with a set value according to FIG. 9.6. FIG. 9.5 shows a circuit for phase reversal.
From DE 2 700 296, the use of opto switches is known in connection with the control of motors as well as the speed control of an electric motor in dependence of the temperature of a system component.
JP 61-234620 describes a switching circuit comprising two anti-parallel connected transistors each connected in series with a blocking diode. The circuit is intended to control the current between an a.c. source and a load.
EP, A1, 381 094 shows a method of coupling phase voltage to a three-phase motor. Switching of the phase voltage takes place via bridge connected thyristors. The control of the switching is accomplished by changing the turn-on angle for the thyristors. Upon e.g. start of the motor the phase voltage is clipped during parts of the period to obtain a winding current which implies a softer start. Then, the turn-on angle can successively be reduced in order to obtain the correct speed. The control signal to one of the two bridge connected thyristors (per phase), is coupled optically.
U.S. Pat. No. 3,866,097 describes a system to couple phase voltage to a three-phase motor. The system differs from that described in EP, A1, 381 094 in that the bridge connections comprise a thyristor and a diode instead of two thyristors.
U.S. Pat. No. 5,070,291 shows an alternative manner to provide a three-phase motor with phase voltage. The motor is supplied with a pseudo-three-phase which is generated by controlling power transistors with pulse width modulated control signals. Instead of coupling an available three-phase signal through, a pseudo-three-phase signal is generated, which has a constant amplitude but a variable frequency.
U.S. Pat. No. 4,716,304 shows a further way of supplying a three-phase motor with a pseudo-three-phase signal. In this example, however, the control signal connected to the power transistors is galvanically separated from the supply voltage.
EP,A1, 357 104 describes the conversion of acoustic signals caused by defects on rotating parts, into electric signals by means of piezoelectric transducers.
EP, A2, 492 739 and U.S. Pat. No. 4,409,548 relate to different methods of measuring rotation speed by means of piezoelectric transducers.
EP,A2, 149 769 describes rotation speed detection by means of optical reading wherein a light modulator in the form a nut is used.
In EP, A1, 221 333 and U.S. Pat. No. 4,132,890 optical reading is described, wherein a disc mounted on the motor shaft is used. The discs have radial equidistant lines which are detected optically.
EP,A1, 66 636 uses also for the same purpose a disc wherein the light beam goes through the disc before it is detected.